Regulation of Tubuloglomerular Feedback by Macula densa-derived Nitric Oxide

Abstract: Studies were conducted to investigate the role of macula densa (MD)-derived nitric oxide (NO) in regulation of the glomerular filtration (GFR) through the tubuloglomerular feedback (TGF) mechanism. Immunocytochemical localization of the neuronal isoform of NO synthase (NOS) in the kidney showed that this isoform was exclusively located in MD cells. Whole kidney clearance measurements of GFR and micropuncture techniques were used to study the importance of MD-NO in the context of TGF on anaesthetized Sprague-Dawley (SD) rats. Systemic infusion of Nw-Nitro-L-Arginine (L-NNA) elevated blood pressure (PA) and proximal tubular stop-flow pressure (PSF), and potently increased TGF sensitivity. Local administration of L-NNA (10-3 M) into the lumen of a single nephron, in which TGF was simultaneously determined, revealed a resetting of TGF sensitivity to that similar after systemic NOS inhibition, but without effects on PA or basal PSF. Similar experiments using specific inhibitors of the different isoforms of NOS that had been localized in the kidney, confirmed that intratubular administration of L-NNA preferably inhibits NOS in the MD cells. Intratubular administration of L-arginine (L-ARG), significantly impaired the TGF response, indicating that MD-NO synthesis can be stimulated by an increased tubular concentration of L-ARG. TGF resetting as a response to L-NNA was abolished in two strains of genetically hypertensive rats, whereas their normotensive control strains responded with increased TGF sensitivity. Oral treatment for 4 weeks with a selective neuronal NOS inhibitor, 7-nitro indazole (7-NI), increased PA in SD rats. But renal excretion and TGF were not different from those in vehicle-treated rats. In acute experiments, 7-NI is as potent as L-NNA in resetting TGF but has no effect on PA. However, TGF is markedly more sensitive and active during the first week of 7-NI-treatment than after week four. Taken together, these results demonstrate that MD-produced NO counteracts TGF-mediated constriction of the afferent arteriole. MD-NO is thus important for maintenance of GFR and renal excretion and thereby for the control of PA.